I tend to avoid Throne Room most of the time. It draws dead (with no action card) no more often than King's Court, of course, but when it doesn't draw dead, its not necessarily a power hand, whereas King's Court-anything normally means you're setting up for a big turn.
Also there's the weaker multiplication:
Throne Room, Throne Room, Action doubled, Action doubled, = four effects from four cards
Throne Room, Throne Room, Action doubled, Throne Room, Action doubled, Action doubled, = six effects from six cards.
King's Court, King Court, Action tripled = three effects from three cards
King's Court, King Court, Action tripled, Action tripled, King Court, Action tripled, Action tripled, Action tripled = fifteen effects from eight cards
So King's Court gets more and more powerful the more of them you have, whereas Throne Room doesn't.
That means that with Throne Room you're not really gaining net effect: the only real benefit is economy of +action, in that you can play two cards for two effects at cost of 1 action rather than 2 actions. In my mind, that makes Throne Room the same as Village: it often replaces itself and you end up one action up. The difference, of course, is when you play Throne Room on a terminal: in that case, its like having played a village already, then getting another copy of the active card, but being obliged to play both those copies immediately.
Where it comes into its own, in my opinion, is the same place that villages come into their own: dense engine decks that expect to play most or all of their deck each turn. King's Court, in contrast, works in any deck that has at least average action density, and is only really excluded when you're looking at Province games with no attacks or +buys and a straight BM+X optimal strategy.